1. Field of the Invention
The invention relates to the control of a continuously variable transmission for a vehicle power train and, more particularly, to the control of clutch slippage in the power train of a vehicle in response to an abrupt increase in demanded power.
2. Description of Prior Art
A continuously variable transmission (CVT) control system typically utilizes a pair of adjustable pulleys, each pulley having at least one sheave that is axially fixed and another sheave that is axially movable relative to the first and a flexible belt of metal or elastomeric material that intercouples the pulleys. The inner faces of the sheaves of the pulley are bevelled or chamfered so that as the axially displaceable sheave moves relative to the fixed sheave, the effective pulley diameter may be adjusted.
Each displaceable sheave includes a fluid constraining chamber for receiving fluid to move the sheave and thus change the effective pulley diameter. As fluid is exhausted from the chamber, the pulley diameter changes in the opposite sense. Generally, the effective diameter of one pulley is moved in one direction as the effective diameter of the other pulley is moved in the other. This enables the ratio between the pulleys to be adjusted. The output of the secondary pulley drives the drive train of the vehicle which in the preferred embodiment is connected to the secondary pulley through a clutch. Reference to U.S. Pat. No. 4,433,594 entitled "Variable Pulley Transmission" assigned to the assignee of this application provides further information in relation to CVTs and is incorporated herein by reference.
Swift vehicle acceleration in response to an abrupt increase in demanded power is a highly desirable characteristic in all vehicles, including those incorporating a continuously variable transmission. To achieve swift acceleration, it is necessary to be able to quickly increase the fuel and air content in the combustion chambers of the engine and also to promote the swift increase in engine speed and enable this to be transferred rapidly to the drive train output.
Vehicles with transmissions having a wide ratio spread, such as continuously variable transmissions, tend to be operated at relatively low engine speed and relatively open throttle. Therefore, although fuel and air can be increased in the engine on demand, the amount of increase available is relatively small. As a consequence, immediately after an abrupt increase in demanded power, the actual engine power increase depends mainly on the rate at which the engine speed can be increased. Also, an engine will be forced to operate under high-torque low speed conditions that can cause surging, uneven firing and excessive exhaust emissions.
A typical response to the problem of high-torque and low speed engine output in CVTs is to provide a control system to vary the ratio in the variable speed component as quickly as possible towards a low ratio. During abrupt increases in the demanded power, the rate of rotation of the CVT output shaft is slowed down transferring some kinetic energy from the vehicle mass to rotation of the crankshaft. In some instances, the ratio can be decreased so fast that in response to an increase in demanded power, the vehicle is suddenly decelerated.
Changing the ratio to remove the problems encountered with surge and uneven firing associated with abrupt increases in power demand, requires an auxiliary control system. However, in many transmission control systems it is difficult to achieve an adequate rate of change without imposing unacceptably high parasitic losses under other operating conditions, such as the steady state. As the periods of operation during which abrupt increases in power are demanded are likely to be comparatively short, the use of such an auxiliary control system can result in a parasitic power loss during the majority of the operating cycle.
It is desirable therefore to provide an alternative manner of improving a vehicle's responsiveness during abrupt changes in demanded power for vehicles having continuously variable transmissions so as not to produce initial vehicle retardation and not to precipitate parasitic power loss during other operating conditions.